Homer is type A and Grandpa (his father) is type O. Marge is type B and her father was type O. The Rh blood group is not linked to the ABO system. It is a two allele group, with positive dominant to negative.
Homer is positive, but his mother is negative. Marge is also negative. What are the chances that Lisa, Homer and Marge's daughter, will be AB negative? (Dihybrid cross)